1. Technical Field
The present invention relates to a diversity reception device with a plural number of branches and a diversity reception method.
2. Related Art
In recent years, orthogonal frequency division multiplexing (OFDM) modulation systems and the like have been employed as modulation systems for terrestrial digital broadcasting. In an OFDM system, symbols are transmitted using plural sub carriers (carrier waves) with different center frequencies. The meaning of the term “symbol” herein is intended to include a single set of data transmitted in one cycle of modulation. A symbol period is constituted by appending a guard period to an effective symbol period. In an OFDM system, the effect of multipath interference is suppressed by copying a portion of an effective symbol signal, which is the actual object of demodulation, and inserting this portion between effective symbol signals in the form of a repeating waveform. The period of this copied waveform is the guard period.
In an OFDM signal receiving device, in order to ameliorate effects from phasing and the like, a diversity system is employed. In this diversity system, plural antennas are provided and carriers are separately demodulated in branches corresponding with the respective antennas. Selection and weighted combining (hereinafter referred to as maximal-ratio combining) or the like of the individually demodulated carriers is executed with reference to reception levels (power levels of the received signals) or the like.
However, when carriers are weighted and combined in accordance with reception levels, the modulation results of branches with reception levels at which reception is not possible, which are actually unwanted, may be combined at large weightings. Thus, the demodulated signal may, contrary to expectation, be worsened and excellent maximal-ratio combining results may not be obtained.
For example, values converted from received signal strength indicator (RSSI) values generated by a tuner and tuner gain control values and the like may be used to represent reception levels and used for weighting in maximal-ratio combining. However, the gain control value applied to an amplifier in a tuner, which is an analog circuit, and the actual gain (i.e., how large the gain is controlled to be in practice) have an error difference. In a diversity receiver with plural branches, because there are differences in actual gains with respect to gain control of the tuners of the respective branches, it is difficult to implement excellent weighting by conversion from RSSI values, gain control values or the like. Further, if unwanted broadcasts or unwanted waves are input to a particular branch, the power level is increased by these unwanted waves in an amount corresponding to the unwanted waves, this is reflected in the gain control, and the combining may be applied with an erroneous weighting.
As a technology for controlling weightings in maximal-ratio combining, Japanese Patent Application Laid-Open (JP-A) No. 2004-135120 has disclosed a diversity reception device that is provided with an automatic gain control (AGC) section that controls the gain of a tuner at each branch, a fast Fourier transform (FFT) section, an equalization section that equalizes the FFT output signal, and a reliability calculation section. The reliability calculation section calculates a reliability value of the carrier from a pilot signal output by the equalization section. The reliability value is corrected by a reliability value correction section in accordance with output information from the AGC section. A carrier selecting and combining section selects and combines, with weightings, the carriers of the branches in accordance with the corrected reliability values.
However, in the diversity reception device recited in the above reference, because the reliability values (weightings) are found using pilot signals output from the equalization section, there is a problem with optimization of the weightings if there is a branch in a receiving condition in which the reception level of wanted waves that are required may not be guaranteed. For example, in this technology, with a received signal in which the wanted waves are weak and unwanted waves are large, the reliability value is found from the level of a pilot signal that has been FFT-processed and equalized. Therefore, the reliability value is affected by reception levels, and maximal-ratio combining with excellent weightings may not be possible.